There's more reason to insulate an aluminum vessel than a stainless one. Aluminum is a great conductor of heat. I keep trying to insulate my aluminum HLT, but my turkey fryer keeps burning the insulation. Oh, for the days of asbestos.

There's little reason (and a some trouble) to insulate a mash tun during heating, but the rests are a different matter. I would build a removable sleeve with foil-backed bubble wrap insulation and duct tape and cover the vesserl with an old blanket or sleeping back during rests.

Well, the aluminum kettle I have in mind is thicker than a turkey fryer. I would be using direct heat. I presume, Bill, that you would reach whatever mash-step you wanted, turn off the flame and slip on the insulation sleeve. Might make it hard to read a thermometer if it was installed in the tun. Lifting the lid off to check temp would cause other temp problems, I suspect.Jim

You might be able to use a thermometer inserted into a thermowell after you slip on the insulating jacket. Another choice would be a digital thermometer with a cable and remote sensor, or even a wireless digital thermometer.

Like many people I mash in a converted keg. I don't worry about insulating it, as I can apply direct heat when necessary.